Loading gauge

A loading gauge defines the maximum height and width for railway vehicles and their loads to ensure safe passage through bridges, tunnels and other structures. Classification systems vary between different countries and gauges may vary across a network, even if the track gauge remains constant.

The loading gauge limits the size of passenger carriages, goods wagons (freight cars) and shipping containers that can be conveyed on a section of railway line and varies across the world and often within a single railway system. Over time there has been a trend towards larger loading gauges and more standardization of gauges with older lines having their structure gauges enhanced by raising bridges, increasing the height of tunnels and making other necessary alterations. Containerisation and a trend towards larger shipping containers has led rail companies to increase structure gauges to compete effectively with road haulage.

The term loading gauge can also refer to a physical structure, sometimes using electronic detectors using light beams on an arm or gantry placed over the exit lines of goods yards or at the entry point to a restricted part of a network. The devices ensure that loads stacked on open or flat wagons stay within the height/shape limits of the line's bridges and tunnels and prevent out-of-gauge rolling stock entering a region with a smaller loading gauge. Compliance with a loading gauge can be checked with a clearance car which in the past were simple wooden frames or physical feelers mounted on rolling stock. More recently lasers are used.

The loading gauge is maximum size of rolling stock, which is distinct from the structure gauge, which is minimum size of bridges and tunnels which must be larger to allow for engineering tolerances and car motion. The difference between the two is called the clearance. The terms dynamic envelope or kinematic envelope – which include factors such as suspension travel, overhang on curves (at both ends and middle) and lateral motion on the track – are sometimes used in place of loading gauge.

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